Capacitors are used in electronic systems for a variety of functions, for example to block direct current while allowing alternating current to pass, smoothing the output of a power supply, reducing interference, and as sources for short, high power pulses of electric energy. A classic capacitor is formed by two conductive plates facing each other but separated by a nonconductive dielectric. When charge is introduced to one plate, an opposite charge forms on the other plate. The amount of energy stored in a capacitor, measured in farads, is dependent upon the surface area of the plates. Classic capacitors therefore grow in size and in mass as they grow in storage capacity. As with all electronics, there is a drive to produce capacitors capable of storing more charge, while decreasing the overall size of the capacitor.
Two types of electrochemical capacitors, electric double layer capacitors (EDLCs) and pseudo-capacitors, have been developed that store considerably more charge than classic capacitors within a given volume. Furthermore, these electrochemical capacitors, because of their high storage capacity, may function similarly to a rechargeable battery, as well as a capacitor. Because of their greatly increased specific energies and capacitances, these types of capacitors are often referred to as supercapacitors.
EDLCs store electrical energy at the interface between an electrolyte and an electrode. Storage of electrical charges is provided by electrostatic accumulation on the surface of the electrode, facilitated by non-Faradaic interaction between the charge of the electrode surface and the ions in the electrolyte. This electrical double layer allows a significant charge to build on the electrode. To increase the size, and thus the capacitance, of an EDLC, the size of the electrode/electrolyte interface is increased by using high surface area electrode materials, such as activated carbon, as the electrode material. Often a sulfuric acid solution serves as the electrolyte. The high surface area per gram provided by activated carbon greatly increases the size of the electric double layer, thus increasing the capacitance of the electrolytic capacitor.
More recently, the discovery of pseudo-capacitance has allowed the manufacture of capacitors with even greater charge storage capacity. Pseudo-capacitance materials undergo Faradaic reactions, reversibly transferring a charge to a material in contact with the electrode in a capacitor. This provides surface-accessible storage of charges at positions spaced from the surface.
Pseudo-capacitance is typically associated with surface reactions based on metal oxides. The metal oxides undergo charging or discharging by valence changes, accepting or surrendering one or more electrons in a Faradaic reaction with the electrode. The metal oxides typically used include ruthenium oxide (RuO2), iridium oxide (IrO2), nickel oxide (NiO), cobalt oxide (CoOx), molybdenum oxide (MoO2) and tungsten oxide (WO3). These materials provide an accessible, reversible pseudo-capacitance in the range of milliFarad per gram to Farad per gram over a 1.4 volt range. Ruthenium oxide (RuO2) has particularly favorable pseudo-capacitance properties. When RuO2 is combined with active carbon electrodes, capacitance of 380 Farads/gram has been achieved.
However, this may be the limit for ruthenium oxide, with a maximum specific energy capacity of about 30 watt hour/kilogram. Often the metal oxide layer of a pseudo-capacitor is crystallized such that RuO2 in the middle of the layer is inaccessible for electron transfer with the electrode, thus limiting the pseudo-capacitance of the material. Further, many suitable metal oxides are expensive and thus impractical for large scale use.
In addition, many pseudo-capacitors require liquid electrolytes to electrostatically balance the charges transferred to the oxide. Aqueous electrolytes can leak and corrode. They also limit the voltage and operating temperature of the capacitor. Solid state electrolytes are known, but they generally have low conductivity resulting in high internal resistance. This prevents a high specific power. Furthermore, the high cost of ruthenium and other suitable metals for the oxide layer hinders the wide application of these devices. Furthermore, many pseudo-capacitors degrade with use and are not suitable for applications requiring durable, long lasting electronics.
It is therefore desirable to provide an energy storage device having a high capacitance and a high specific energy. It is also desirable to provide a durable, high power energy storage device that does not require a liquid electrolyte. It is also desirable to provide a durable, high power energy storage device manufactured from inexpensive materials.